The present invention relates to the use of supercritical or near supercritical carbon dioxide to remove sticky contaminants (hereinafter referred to as "stickies") from secondary fibers.
As known in the art, secondary fibers comprise materials, usually cellulose-based, which have been used at least once in their intended primary use area but are, nevertheless, amenable to further processing and subsequent reuse. Waste paper, newsprint, ledger stock, packaging materials, cartons, boxes, computer printouts, telephone directories, corrugated boards, and the like represent suitable raw stock for conversion to secondary fibers. The pattern of reuse (i.e., use of the secondary fiber) may not always be similar to the use to which the primary (virgin) fiber was put.
Efficient management of solid wastes, of which cellulose-based materials constitute a significant part (e.g., waste paper, 40%; yard waste, 18%), has become an important societal theme. In recent years, efforts to recycle waste paper have intensified with the ever increasing concerns as to the rate of use of raw materials and the possible adverse environmental impact of common industrial processes. Novel screening systems and sophisticated flotation techniques have emerged which in large measure have successfully addressed the problem of de-inking printed stock. New bleaching sequences which avoid the use of chlorine or chlorine compounds and rely solely upon hydrogen peroxide, dithionites, or formamidine sulfinic acid for attaining acceptable levels of brightness are also making their appearance.
One aspect of waste paper reuse, however, has remained a continuing problem both with regard to machine operability and with regard to product quality. This area is the presence of sticky contaminants in the stock used to prepare the secondary fibers.
Stickies consist primarily of organic adhesives used in the paper converting industry, such as, hot melts, pressure-sensitive adhesives, styrofoam, and latices. Typical stickies include: polyvinyl acetate (PVA) polymers and copolymers, ethylene vinyl acetate (EVA) polymers and copolymers, polystyrene, styrene-butadiene, polypropylene, polyethylene, polyamide, latex and other rubber compounds, wax, and the like. A particularly common source of stickies is the tackifiers which are added to paper products to improve adhesion properties.
When waste paper containing these adhesives/tackifiers are defibered, stickies are broken down into particles having a wide range of sizes, e.g., less than 0.074 millimeters to greater than 0.42 millimeters. Inefficient removal of stickies causes off-quality paper (e.g., poor appearance, lower strength, and/or holes) and paper machine downtime (e.g., web breaks, slippery sheets, and/or deposition of stickies on such machine components as wires, felts, presses, rolls, and/or drying cylinders).
In an effort to deal with these problems various chemical and mechanical means have been considered. For example, talc and zirconium oxide have been used as pacification agents for stickies. High temperatures, high pressures, and/or solvent addition have been employed as dispersion techniques. Reverse hydroclones and throughflow cleaners have been used to try to screen and/or clean stickies from the feed stock.
Also, efforts have been made to select the stock used for recycling ("furnish selection"). In general, furnish selection is not cost effective. Also, as fiber recycling becomes more important in the context of environmental concerns, the furnish itself will tend to become less "virgin" and more of secondary and tertiary origin, which will, in turn, seriously aggravate the stickies problem.
Notwithstanding these wide ranging efforts, the removal of stickies remains an unsolved problem in the area of fiber recycling. As recognized in the art, irrespective of the technique used, some stickies end up in the paper machine headbox where they adversely affect machine runability and product quality. Accordingly, there is an important and continuing need to remove or substantially reduce the content of stickies in waste paper stock which is going to be recycled. It is an object of the present invention to respond to this need.
Various uses of supercritical fluids in the processing of materials have been disclosed in the literature. For example, supercritical carbon dioxide has been used to remove tall oil and turpentine from coniferous woods in Fremont, U.S. Pat. No. 4,308,200, to extract lignin from the black liquor produced by the Kraft process for pulp production in Avedesian, U.S. Pat. No. 4,493,797, to treat refinery sludges in European Patent Publication No. 314,223, to regenerate absorbents used in waste water treatment systems in Modell, U.S. Pat. Nos. 4,061,566 and 4,147,624, to sterilize pharmaceuticals in Pilz et al., U.S. Pat. No. 4,263,253, to remove off-flavor materials from textured vegetable products in Sevenants, U.S. Pat. No. 4,675,198, to remove gamma-linolenic acid from fruit seeds in Traitler et al., U.S. Pat. No. 4,703,060, and to decaffeinate coffee in Katz, U.S. Pat. No. 4,472,442; Toro et al., U.S. Pat. No. 4,728,525 and Kaleda et al., U.S. Pat. No. 4,767,634. See also, Friedrich, U.S. Pat. No. 4,466,923; Lawson et al., U.S. Pat. No. 4,495,095; Myerson, U.S. Pat. No. 4,550,198; Panzner et al., U.S. Pat. 4,554,170; Japikse et al., U.S. Pat. No. 4,647,466; Ritter and Campbell, "The Effects of Supercritical Carbon Dioxide Extraction on Pine Wood Structure", Biotechnology and Bioengineering Symp., 1986, no. 17, pp. 179-182; Hatakeda et al., "Extraction of Sugi (Cryptomeria japonica D. Don) with supercritical carbon dioxide", Nipon Kagaku Kaishi, 1987, no. 5, pp. 931-933; Shishikura et al., "Concentration of Tocopherols from Soybean Sludge by Supercritical Fluid Extraction", J. Jpn. Oil Chem. Soc., 1988, vol. 37, pp. 8-12; and Li and Kiran "Interaction of Supercritical Fluids with Lignocellulosic Materials", Ind. Eng. Chem. Res., 1988, vol. 27, pp. 1301-1312.
Similarly, supercritical water or near supercritical water has been used to treat wood chips and black liquor from pulping in Modell, PCT Patent Publication No. WO 81/00855. See also Modell M., "Gasification and Liquefaction of Forest Products in Supercritical Water", Fundam. Thermochem. Biomass Convers., 1985, pp. 95-119; and West et al., "Pyrolysis of 1,3-butanediol as a model reaction for wood liquefaction in supercritical water", Can. J. Chem. Eng., 1987, vol. 65, pp. 645-650.
A review of the use of supercritical fluids, including supercritical carbon dioxide in various industrial and pollution control processes can be found in Eckert et al., "Supercritical fluid processing", Environ. Sci. Technol., 1986, vol 20, pp. 319-325. Other studies involving the use of supercritical fluids including carbon dioxide to remove organic materials, in particular, hazardous organic materials, from environmental solids can be found in Pang et al., "Supercritical Extraction of Aromatic Hydrocarbon Solids and Tar and Bitumens", Ind. Eng. Chem. Process. Des. Dev., 1985, vol. 24, pp. 1027-1032, Groves et al. "State-of-the-art on the supercritical extraction of organics from hazardous wastes", CRC Critical Reviews in Environmental Control, 1985, vol. 15, pp. 237-274; Hawthorne et al., "Extraction and Recovery of Polycyclic Aromatic Hydrocarbons from Environmental Solids Using Supercritical Fluids", Anal. Chem., 1987, vol. 59, pp. 1705-1708; Dooley et al., "Supercritical Fluid Extraction and Catalytic Oxidation of Toxic Organics from Soils", EPA Report No. 600/9-87/018F, pp. 383-397; and Brady et al. "Supercritical Extraction of Toxic Organics from Soils", Ind. Eng. Chem. Res., 1987, vol. 26, pp. 261-268.
Along these same lines, U.S. Pat. Nos. 4,338,199 and 4,543,190 to Modell describe a process in which organic materials are oxidized in supercritical water. The '199 patent includes a general statement that its process can be used to remove toxic chemicals from the wastes generated by a variety of industries including forest product wastes and paper and pulp mill wastes. The use of supercritical water to treat organic waste materials is also disclosed in PCT Patent Publication No. WO 81/00854, Modell et al., U.S. Pat. No. 4,113,446, Burleson, U.S. Pat. No. 4,564,458, and Titmas, U.S. Pat. No. 4,594,164. A summary of experiments performed by Modar, Inc., using the Modell supercritical water process was published in Chemosphere--Chlorinated Dioxins and Related Compounds 1987, McNelis et al., editors, Pergamon Press, New York, 1989, Vol. 18, Nos. 1-6, page 50.
In addition to their use in materials processing and waste treatment, supercritical fluids have been used in connection with various analytic procedures. For example, Suprex Publication No. TN-022, Suprex Corporation, Pittsburgh, Pa., 1989, mentions the use of supercritical carbon dioxide as part of an analytical procedure for assaying dioxins. Similarly, Hawthorne et al., "Directly coupled supercritical fluid extraction-gas chromatographic analysis of polycyclic aromatic hydrocarbons and polychlorinated biphenyls from environmental solids" J. Chromatogr., 1987, vol. 403, pp. 63-76, discuss the use of supercritical fluid extraction coupled to a gas chromatograph to analyze environmental solids, e.g., urban dust, for organic pollutants, specifically, polycyclic aromatic hydrocarbons. Along similar lines, Schneiderman et al., "Determination of anthraquinone in paper and wood using supercritical fluid extraction and high-performance liquid chromatography with electrochemical detection", J. Chromatogr., 1987, 409, pp. 343-353, describe the combination of supercritical fluid extraction using carbon dioxide, high-performance liquid chromatography, and electrochemical detection to analyze Kraft paper and pine plywood sawdust for anthraquinone.
Significantly, none of these references in any way discloses or suggests that the problem of removing stickies from secondary fibers can be solved by treating the fibers with supercritical carbon dioxide.